Virtualtype

Declare virtual methods

Syntax

Type typename Extends base_typename

Declare Virtual Sub|Function|Property|Operator|Destructor ...

End Type

Description

Virtual methods are methods that can be overridden by data types derived from the type they were declared in, allowing for dynamic polymorphism. In contrast to Abstract methods, virtual methods must have an implementation, which is used when the virtual is not overridden.

A derived type can override virtual methods declared in its base type by declaring a non-static method with the same identifier and signature, meaning same number and type of parameters (invariant parameters), same calling convention, and if any, same return type (or a covariant return type for return by reference or by pointer):

• if that differs only in parameter passing mode or calling convention or return type, then an overriding error is returned at compile time,
  
• otherwise, only shadowing is allowed for any other signature difference, corresponding to case where both methods would be overloadable (if within the same type).
  

The property of being a virtual method is not implicitly inherited by the overriding method in the derived type. If this overriding method must be overridden in turn in a lower level derived type, it must also be declared as virtual.
On the other hand, since a derived static method can never override a base virtual/abstract method, it can therefore shadow any base method (including virtual/abstract) with same identifier and regardless of the signature.

When calling virtual methods, the compiler may need to do a vtable lookup in order to find out which method must be called for a given object. This requires an extra hidden vtable pointer field to be added at the top of each type with virtual methods. This hidden vptr is provided by the built-in Object type. Because of that, virtual methods can only be declared in a type that directly or indirectly Extends Object.

Dynamic polymorphism by using override procedures:

• Normally only a typename procedure (or upper in hierarchy) is accessible through a base-typename reference/pointer even if this one refers to an object derived from typename.
  
• But when the procedure is virtual, this tells the running program to resolve the override procedure the most derived relating to the real object type by vtable lookup (dynamic binding at runtime), rather than procedure normally accessible from the raw base-type of the reference/pointer (static binding at compile time).
  
  

Constructors cannot be virtual because they create objects, while virtual methods require an already-existing object with a specific type. The type of the constructor to call is determined at compile-time from the code.
In addition, when calling a virtual method inside a constructor, only the version of the method corresponding to an object of type of this constructor is used. That is because the vptr has not yet been set up by the derived type constructor, but only by the local type constructor.

Destructors often must be virtual when deleting an object manipulated through a pointer to its base type, so that the destruction starts at the most derived type and works its way down to the base type. To do this, it may be necessary to add virtual destructors with an empty body anywhere an explicit destruction was not yet required, in order to supersede each non-virtual implicit destructor built by the compiler.
On the other hand, when calling a virtual (or abstract) method inside a destructor (virtual or not), only the version of the method corresponding to an object of type of this destructor is used because the vptr is reset at the top of the destructor according to its own type's vtable. This avoids to access child methods and so to refer to child members previously destroyed by the child destructor execution.

For member methods with Virtual in their declaration, Virtual can also be specified on the corresponding method bodies, for improved code readability.

Caution: In a multi-level inheritance, a same named method (same identifier and signature) can be declared Abstract, Virtual or normal (without specifier) at each inheritance hierarchy level. When there is mixing of specifiers, the usual order is abstract -> virtual -> normal, from top to bottom of the inheritance hierarchy.
The access control (Public/Protected/Private) of an overriding method is not taken into account by the internal polymorphism process, but only for the initial call at compile-time.
Base (Member Access).method() calls always the base's own method, never the overriding method.

Note: Virtual is also used as qualifier for ProcPtr to request it to return the index in the vtable for a virtual/abstract member procedure or member operator.

Remarks

Dialect Differences

• Only available in the -lang fb dialect.
  
  

Differences from QB

• New to FreeBASIC
  
  

See also

• Type
  
• Object
  
• Extends
  
• Extends Zstring
  
• Extends Wstring
  
• Abstract
  
• Override
  
  

Example

'' Example with overriding subroutines





Type Hello Extends Object

    Declare Virtual Sub hi( )

End Type



Type HelloEnglish Extends Hello

    Declare Sub hi( )            '' overriding subroutine

End Type



Type HelloFrench Extends Hello

    Declare Sub hi( )            '' overriding subroutine

End Type



Type HelloGerman Extends Hello

    Declare Sub hi( )            '' overriding subroutine

End Type





Sub Hello.hi( )

    Print "hi!"

End Sub



Sub HelloEnglish.hi( )           '' overriding subroutine

    Print "hello!"

End Sub



Sub HelloFrench.hi( )            '' overriding subroutine

    Print "Salut!"

End Sub



Sub HelloGerman.hi( )            '' overriding subroutine

    Print "Hallo!"

End Sub





Randomize( Timer( ) )



Dim As Hello Ptr h



For i As Integer = 0 To 9

    Select Case( Int( Rnd( ) * 4 ) + 1 )

    Case 1

        h = New HelloEnglish

    Case 2

        h = New HelloFrench

    Case 3

        h = New HelloGerman

    Case Else

        h = New Hello

    End Select



    h->hi( )

    Delete h

Next



Sleep

'' Example with overriding destructor and

''              overriding function with covariant return





Type myBase Extends Object

  Declare Virtual Function clone () As myBase Ptr

  Declare Virtual Sub Destroy ()

End Type



Function myBase.clone () As myBase Ptr

  Dim As myBase Ptr pp = New myBase(This)

  Print "myBase.clone() As myBase Ptr", pp

  Function = pp

End Function



Sub myBase.Destroy ()

  Print "myBase.Destroy()", , @This

  Delete @This

End Sub





Type myDerived Extends myBase

  Declare Function clone () As myDerived Ptr     '' overriding member function with covariant return

  Declare Sub Destroy ()                         '' overriding member subroutine

End Type



Function myDerived.clone () As myDerived Ptr     '' overriding member function with covariant return

  Dim As myDerived Ptr pc = New myDerived(This)

  Print "myDerived.clone() As myDerived Ptr", pc

  Function = pc

End Function



Sub myDerived.Destroy ()                         '' overriding member subroutine

  Print "myDerived.Destroy()", , @This

  Delete @This

End Sub





Dim As myDerived c



Dim As myBase Ptr ppc = @c

Dim As myDerived Ptr pcc = @c



Dim As myBase Ptr ppc1 = ppc->clone()            '' using base pointers and polymorphism

Dim As myDerived Ptr pcc1 = pcc->clone()         '' using derived pointers and covariance of return value

Print

ppc1->Destroy()                                  '' using base pointer and polymorphism

pcc1->Destroy()                                  '' using derived pointer



Sleep

Reference

• Documented in KeyPgVirtual.html